Different Responses in Mandarin Cultivars Uncover a Role of Cuticular Waxes in the Resistance to Citrus Canker

Clarification of the infection pattern of Xanthomonas citri subsp. citri on citrus fruit by artificial inoculation

BACKGROUND

Citrus canker is a significant bacterial disease caused by Xanthomonas citri subsp. citri (Xcc) that severely impedes the healthy development of the citrus industry. Especially when citrus fruit is infected by Xcc, it will reduce or even lost its commercial value. However, due to the prolonged fruiting cycle and intricate structure, much less research progress had been made in canker disease on fruit than on leaf. In fact, limited understanding has been achieved on canker development and the response to Xcc infection in fruit.

RESULTS

Herein, the progression of canker disease on sweet orange fruit was tracked in the field. Results indicated that typical lesions initially appear on the sepal, style residue, nectary disk, epicarp, and peduncle of young fruits after petal fall. The susceptibility of fruits to Xcc infection diminished as the fruit developed, with no new lesions forming at the ripening stage. The establishment of an efficient method for inoculating Xcc on fruit as well as the artificial inoculation throughout the fruit's developmental cycle clarified this infection pattern. Additionally, microscopic observations during the infection process revealed that Xcc invasion caused structural changes on the surface and cross-section of the fruit.

CONCLUSIONS

An efficient system for inoculation on citrus fruit with Xcc was established, by which it can serve for the evaluation of citrus germplasm for canker disease resistance and systematic research on the interactions between Xcc and citrus fruits.

Priming crop plants with rosemary (Salvia rosmarinus Spenn, syn Rosmarinus officinalis L.) extract triggers protective defense response against pathogens

Plant bioactive compounds provide novel straightforward approaches to control plant diseases. Rosemary (Salvia rosmarinus)-derived extracts carry many prominent pharmacological activities, including antimicrobial and antioxidant, mainly due to its phenolic compounds, rosmarinic acid (RA), carnosic acid and carnosol. However, the effects of these extracts on plant diseases are still unknown, which constrains its potential application as bioprotectant in the agricultural production. In this study we demonstrate the antiviral effect of the aqueous rosemary extract (ARE) against tobacco necrosis virus strain A (TNVA) in ARE-treated tobacco (Nicotiana tabacum) plants. Our results show that ARE-treatment enhances plant defense response, contributing to reduce virus replication and systemic movement in tobacco plants. RA, the main phenolic compound detected in this extract, is one of the main inducers of TNVA control. The ARE-induced protection in TNVA-infected plants was characterized by the expression of H₂O₂ scavengers and defense-related genes, involving salicylic acid- and jasmonic acid-regulated pathways. Furthermore, treatment with ARE in lemon (Citrus limon) and soybean (Glycine max) leaves protects the plants against Xanthomonas citri subsp. citri and Diaporthe phaseolorum var. meridionalis, respectively. Additionally, ARE treatment also promotes growth and development, suggesting a biostimulant activity in soybean. These results open the way for the potential use of ARE as a bioprotective agent in disease management.

Epicuticular wax of sweet sorghum influenced the microbial community and fermentation quality of silage

Epicuticular wax, as secondary metabolites (hydrophobic compounds) covering plant surface, plays important roles in protecting plants from abiotic and biotic stresses. However, whether these compounds will influence fermentation process of silage is still not clear. In this study, two sweet sorghum cultivars with varying epicuticular wax on sheath (bloom), Yajin 2 (YJ, less bloom), and Jintian (JT, dense bloom), were harvested at flowering and maturing stages, and ensiled with or without bloom, aiming to evaluate the effects of bloom on fermentation quality, feed nutrition and microbial community. The bloom was collected manually with de-waxed cotton and extracted with chloroform. The results showed that the bloom reduced the concentrations of water-soluble carbohydrate and crude protein of the two cultivars at both stages, reduced lactic acid (LA) for YJ at both stages and for JT at flowering stage, and increased LA for JT at mature stage. The α-diversity of bacterial communities of the silage fermentation with bloom was significantly lower than that without bloom. Bloom increased the abundance of Lactobacillus, reduced that of Bacillus and Weissella, and significant correlations were observed between fermentative qualities and bacterial abundances. However, decreased diversity of bacterial community and the contents of LA implied that shifts in bacterial community might exert negative effects on silage fermentation. Our results suggest that bloom wax could alter the microbial community composition of ensiled sweet sorghums, which thus influence the fermentation qualities.

Rapid Evaluation of the Resistance of Citrus Germplasms Against Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri (Xcc) is the causal agent of citrus bacterial canker (CBC), one of the most devastating citrus diseases. Most commercial citrus varieties are susceptible to CBC. However, some citrus varieties and wild citrus germplasms are CBC resistant and are promising in genetic increases in citrus resistance against CBC. We aimed to evaluate citrus germplasms for resistance against CBC. First, we developed a rapid evaluation method based on enhanced yellow fluorescent protein (eYFP)-labeled Xcc. The results demonstrated that eYFP does not affect the growth and virulence of Xcc. Xcc-eYFP allows measurement of bacterial titers but is more efficient and rapid than the plate colony counting method. Next, we evaluated citrus germplasms collected in China. Based on symptoms and bacterial titers, we identified that two citrus germplasms ('Ichang' papeda and 'Huapi' kumquat) are resistant, whereas eight citrus germplasms ('Chongyi' wild mandarin, 'Mangshan' wild mandarin, 'Ledong' kumquat, 'Dali' citron, 'Yiliang' citron, 'Longyan' kumquat, 'Bawang' kumquat, and 'Daoxian' wild mandarin) are tolerant. In summary, we have developed a rapid evaluation method to test the resistance of citrus plants against CBC. This method was successfully used to identify two highly canker-resistant citrus germplasms and eight citrus germplasms with canker tolerance. These results could be leveraged in traditional breeding contexts or be used to identify canker resistance genes to increase the disease resistance of commercial citrus varieties via biotechnological approaches.

Biallelic Editing of the LOB1 Promoter via CRISPR/Cas9 Creates Canker-Resistant 'Duncan' Grapefruit

Citrus canker caused by Xanthomonas citri subsp. citri is one of the most devastating citrus diseases worldwide. Generating disease-resistant citrus varieties is considered one of the most efficient and environmentally friendly measures for controlling canker. X. citri subsp. citri causes canker symptoms by inducing the expression of canker susceptibility gene LOB1 via PthA4, a transcription activator-like (TAL) effector, by binding to the effector binding element (EBE) in the promoter region. In previous studies, canker-resistant plants were generated by mutating the coding region or the EBE of LOB1. However, homozygous or biallelic canker-resistant plants have not been generated for commercial citrus varieties, such as grapefruit (Citrus paradisi), which usually contain two alleles of LOB1 and thus, have two types of LOB1 promoter sequences: TI LOBP and TII LOBP. Two different sgRNAs were used to target both EBE types. Both 35S promoter and Yao promoter were used to drive the expression of SpCas9p to modify EBE_PthA4-LOBP in grapefruit. Using 'Duncan' grapefruit epicotyls as explants, 19 genome-edited grapefruit plants were generated with one biallelic mutant line (#Dun_Yao7). X. citri subsp. citri caused canker symptoms on wild-type and nonbiallelic mutant plants but not on #Dun_Yao7. XccPthA4 mutant containing the designer TAL effector dLOB1.5, which recognizes a conserved sequence in both wild-type and #Dun_Yao7, caused canker symptoms on both wild-type and #Dun_Yao7. No off-target mutations were detected in #Dun_Yao7. This study represents the first time that CRISPR-mediated genome editing has been successfully used to generate disease-resistant plants for 'Duncan' grapefruit, paving the way for using disease-resistant varieties to control canker.

Highly Efficient Generation of Canker-Resistant Sweet Orange Enabled by an Improved CRISPR/Cas9 System

Sweet orange (Citrus sinensis) is the most economically important species for the citrus industry. However, it is susceptible to many diseases including citrus bacterial canker caused by Xanthomonas citri subsp. citri (Xcc) that triggers devastating effects on citrus production. Conventional breeding has not met the challenge to improve disease resistance of sweet orange due to the long juvenility and other limitations. CRISPR-mediated genome editing has shown promising potentials for genetic improvements of plants. Generation of biallelic/homozygous mutants remains difficult for sweet orange due to low transformation rate, existence of heterozygous alleles for target genes, and low biallelic editing efficacy using the CRISPR technology. Here, we report improvements in the CRISPR/Cas9 system for citrus gene editing. Based on the improvements we made previously [dicot codon optimized Cas9, tRNA for multiplexing, a modified sgRNA scaffold with high efficiency, citrus U6 (CsU6) to drive sgRNA expression], we further improved our CRISPR/Cas9 system by choosing superior promoters [Cestrum yellow leaf curling virus (CmYLCV) or Citrus sinensis ubiquitin (CsUbi) promoter] to drive Cas9 and optimizing culture temperature. This system was able to generate a biallelic mutation rate of up to 89% for Carrizo citrange and 79% for Hamlin sweet orange. Consequently, this system was used to generate canker-resistant Hamlin sweet orange by mutating the effector binding element (EBE) of canker susceptibility gene CsLOB1, which is required for causing canker symptoms by Xcc. Six biallelic Hamlin sweet orange mutant lines in the EBE were generated. The biallelic mutants are resistant to Xcc. Biallelic mutation of the EBE region abolishes the induction of CsLOB1 by Xcc. This study represents a significant improvement in sweet orange gene editing efficacy and generating disease-resistant varieties via CRISPR-mediated genome editing. This improvement in citrus genome editing makes genetic studies and manipulations of sweet orange more feasible.

Examination of the Global Regulon of CsrA in Xanthomonas citri subsp. citri Using Quantitative Proteomics and Other Approaches

The RNA-binding protein CsrA is a global posttranscriptional regulator and controls many physiological processes and virulence traits. Deletion of csrA caused loss of virulence, reduced motility and production of xanthan gum and substantial increase in glycogen accumulation, as well as enhanced bacterial aggregation and cell adhesion in Xanthomonas spp. How CsrA controls these traits is poorly understood. In this study, an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomic analysis was conducted to compare the protein profile of wild-type strain Xanthomonas citri subsp. citri and the isogenic ΔcsrA strain. A total of 2,374 proteins were identified, and 284 were considered to be differentially expressed proteins (DEP_S), among which 151 proteins were up-regulated and 133 were down-regulated in the ΔcsrA strain with respect to the wild-type strain. Enrichment analysis and a protein-protein interaction network analysis showed that CsrA regulates bacterial secretion systems, flagella, and xanthan gum biosynthesis. Several proteins encoded by the gumB operon were down-regulated, whereas proteins associated with flagellum assembly and the type IV secretion system were up-regulated in the ΔcsrA strain relative to the Xcc306 strain. These results were confirmed by β-glucuronidase assay or Western blot. RNA secondary structure prediction and a gel-shift assay indicated that CsrA binds to the Shine-Dalgarno sequence of virB5. In addition, the iTRAQ analysis identified 248 DEPs that were not previously identified in transcriptome analyses. Among them, CsrA regulates levels of eight regulatory proteins (ColR, GacA, GlpR, KdgR, MoxR, PilH, RecX, and YgiX), seven TonB-dependent receptors, four outer membrane proteins, and two ferric enterobactin receptors. Taken together, this study greatly expands understanding of the regulatory network of CsrA in X. citri subsp. citri.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

PthAW1, a Transcription Activator-Like Effector of Xanthomonas citri subsp. citri, Promotes Host-Specific Immune Responses

Citrus canker disease caused by Xanthomonas citri subsp. citri is one of the most destructive diseases in citrus. X. citri subsp. citri pathotypes display different host ranges. X. citri subsp. citri strain A (Xcc^A) causes canker disease in most commercial citrus varieties, whereas strain AW (Xcc^AW), which is genetically similar to Xcc^A, infects only lime and alemow. Understanding the mechanism that determines the host range of pathogens is critical to investigating and utilizing host resistance. We hypothesized that Xcc^AW would undergo mutations in genes that restrict its host range when artificially inoculated into incompatible citrus varieties. To test this hypothesis, we used an experimental evolution approach to identify phenotypic traits and genetic loci associated with the adaptation of Xcc^AW to incompatible sweet orange. Repeated inoculation and reisolation cycles improved the ability of three independent Xcc^AW strains to colonize sweet orange. Adapted Xcc^AW strains displayed increased expression of type III secretion system and effector genes. Genome sequencing analysis indicated that two of the adapted strains harbored mutations in pthAW1, a transcription activator-like effector (TALE) gene, that corresponded to the removal of one or two repeats from the central DNA-binding repeat region. Introduction of the original but not the adapted pthAW1 variants into Xcc^A abolished its ability to cause canker symptoms in sweet orange, Meyer lemon, and clementine but not in other Xcc^AW-resistant citrus varieties. The original pthAW1, when expressed in Xcc^A, induced ion leakage and the expression of pathogenesis-related genes but had no effect on CsLOB1 expression in sweet orange. Our study has identified a novel host-specific avirulence TALE and demonstrated active adaptive rearrangements of the TALE repeat array during host adaptation.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Diversity and Geographical Structure of Xanthomonas citri pv. citri on Citrus in the South West Indian Ocean Region

A thorough knowledge of genotypic and phenotypic variations (e.g., virulence, resistance to antimicrobial compounds) in bacteria causing plant disease outbreaks is key for optimizing disease surveillance and management. Using a comprehensive strain collection, tandem repeat-based genotyping techniques and pathogenicity assays, we characterized the diversity of X. citri pv. citri from the South West Indian Ocean (SWIO) region. Most strains belonged to the prevalent lineage 1 pathotype A that has a wide host range among rutaceous species. We report the first occurrence of genetically unrelated, nonepidemic lineage 4 pathotype A* (strains with a host range restricted to Mexican lime and related species) in Mauritius, Moheli and Réunion. Microsatellite data revealed that strains from the Seychelles were diverse, grouped in three different clusters not detected in the Comoros and the Mascarenes. Pathogenicity data suggested a higher aggressiveness of strains of one of these clusters on citron (Citrus medica). With the noticeable exception of the Comoros, there was no sign of recent interisland movement of the pathogen. Consistent with this finding, the copL gene, a marker for the plasmid-borne copLAB copper resistance that was recently identified in Réunion, was not detected in 568 strains from any islands in the SWIO region apart from Réunion.

Update on Cuticular Wax Biosynthesis and Its Roles in Plant Disease Resistance

The aerial surface of higher plants is covered by a hydrophobic layer of cuticular waxes to protect plant tissues against enormous environmental challenges including the infection of various pathogens. As the first contact site between plants and pathogens, the layer of cuticular waxes could function as a plant physical barrier that limits the entry of pathogens, acts as a reservoir of signals to trigger plant defense responses, and even gives cues exploited by pathogens to initiate their infection processes. Past decades have seen unprecedented proceedings in understanding the molecular mechanisms underlying the biosynthesis of plant cuticular waxes and their functions regulating plant–pathogen interactions. In this review, we summarized the recent progress in the molecular biology of cuticular wax biosynthesis and highlighted its multiple roles in plant disease resistance against bacterial, fungal, and insect pathogens.